The present invention relates to a flexible electro-luminescent light source, which is particularly useful for operation under the conditions of high humidity.
Electro-luminescent light sources are widely used due to the fact that they are relatively long-lived, have a low power consumption and emit bright fight. However, they are known to be very sensitive to moisture. The penetration of moisture inside such a light source and its interaction with electro-luminescent layers result in changes in their electro-optical characteristics. In particular, effects, such as the reduction in brightness of their luminescence and an increase in capacitance and current leakage, are observed. All these phenomena reduce the lifetime of the electro-luminescent light source.
Various techniques have been developed aimed at prong electro-luminescent layers in light sources from moisture penetration. The techniques of one kind are based on the micro-capsulation or the powders of electro-luminescent materials. According to this approach, each particle of the electro-luminescent powder is provided with a protective layer, e.g., oxide, that protects the particle from interacting with the molecules of water. Such techniques are disclosed, for example, in the following U.S. Pat. Nos. 5,418,062; 5,244,750; 5,220,243; 4,902,929 and 4,855,189.
Techniques of another kind are associated with the protection of the entire light source from moisture penetration thereinside. For example, light sources sealed in glass or metal/glass housings, can practically be sufficiently protected. However, these light sources cannot be made flexiblexe2x80x94they are heavy, and their dimensions and shape are limited by the lousing material. These features significantly restrict the field of applications of such light sources.
For sealing flexible electro-luminescent light sources, various barrier layers have been used, for example, viscose silicone oil or grease that cover the surfaces prior to depositing outer polymer coatings (U.S. Pat. No. 5,869,930), and transparent flexible polymer materials with low permeability for water steams, e.g., various fluoropolymers. The techniques of this kind are disclosed in the following U.S. Pat. Nos. 5,959,402; 5,770,920; 4,455,324 and 4,417,174.
A constructional element made of materials capable of absorbing moisture can also be used for protecting light sources from moisture penetration. This approach is disclosed, for example, in U.S. Pat. Nos. 5,869,930 and 5,959,402.
U.S. Pat. No. 5,976,613 discloses a thick film electro-luminescent film and a method of its manufacture, aimed at solving the moisture problem. According to this technique, a non-hydroscopic binder is used for both phosphor and adjacent dielectric layers, thereby obviating the need for an external protective encapsulation.
All the known methods of protecting flexible electro-luminescent light sources from moisture penetration are passive, and moisture penetrates inside the light source when it is maintained at an atmosphere with a relative humidity of more than 80%. This results in that the electro-optical characteristic of such a light source changes, and correspondingly, its lifetime is significantly reduced. Moisture affects a light source, especially when it is in its inoperative, passive mode (does not emit light), which is the typical case at bright external illumination:
There is accordingly a need in the art to solve the problem of prolonging the lifetime of electro-luminescent light sources operating under the conditions of high humidity, by providing a novel electro-luminescent light source formed with an electrical heating element.
There is provided, according to a broad aspect of the present invention, a substantially flexible, electro-luminescent light source comprising:
(a) an electrodes"" assembly;
(b) dielectric and electro-luminescent layers;
(c) at least one outer, substantially flexible layer formed of insulating transparent material;
(d) a heating element; and
(e) a power supply unit coupled to said electrodes"" assembly and to said heating element for selectively operating thereof, such as to heat the vicinity of the electrodes"" assembly thereby maintaining desired temperature conditions in the vicinity of the light source and thereinside.
The heating element is preferably flexible, based for example on deposited conductive layers or wires. The heating element is accommodated in the light source in such a manner that, when it is switched on, the light source is heated all along its area. The heating of the light source results in that relative humidity of air is reduced in the vicinity of the light source and in pores and cavities thereinside. In other words, the light source becomes located in the dryer air. With the reduction in the relative humidity of air that surrounds the light source and is located thereinside, the probability of interaction between the electro-luminescent material with the water steams reduces. In practice, to achieve the significant effect, heating at a temperature of 4-6xc2x0 C. is sufficient.
If the heating element does not extend along the entire surface of the emitting layer, which is the typical case for a wire-like heating element, the heating element is accommodated in a flexible, heat conductive layer. This provides the temperature balancing along the entire surface of the light source. For example, a layer of viscous polyetilenglicol mixed with Sodium Lauryl Sulfatic can be used as the heat conductive layer.
The heating element can be an additional conductive layer. The heating element can be made from special wire elements of the construction, or from conductive grids introduced to the light source construction, especially for this purpose. One of the electro-conductive layers typically existing in the construction of a light source, or any other conductive elements of the construction can function as the heating element, provided it is appropriately connected to a power supply unit. More specifically, a transparent electrode, an opaque (rear) electrode, as well lo as a contact to the transparent electrode, can serve as the heating element. Preferably, when the light source is in its passive operational mode, the heating element is necessarily connected to the power supply unit (i.e., is in its operative mode), and may and may not be connected to the power supply unit, when in the active operational mode of the light source. However, the heating element can be continuously connected to the power supply unit, irrespectively of the operational mode of the light source. This is the simplest and the cheapest example. In practice, such a continuous operational mode of the heating element is required at very low voltages and frequencies, when the light source is hardly luminescent. It is clear that with this active mode of the light source, the heat liberation will be negligible, and therefore the heating element should be continuously turned on.
The heating element can be connected to the power supply unit through a switch that actuates and disactuates the current flow through the heating element, depending on the operational mode of the light source. More specifically, when the light source is in its active operational mode, i.e., emits light, the heating element is disconnected from the current source, and when the light source is in its passive operational mode, i.e., does not emit light, the heating element is connected to the current source. This kind of operation is preferable in such cases, when, in the active operational mode of the light source, the conversion of electrical energy into light energy in the light source is followed by heat dissipation sufficient for raising the temperature of the surface of the emitting layer to 5-10xc2x0 C. higher than the temperature of the surroundings. In this case, heating in the active operational mode of the light source is not needed, and may even be harmful, since it may reduce the lifetime of the light source. The switch actuating and disactuating the current flow through the heating element may comprise a photo-sensitive element. At bright illumination, when an electro-luminescent light source is ineffective, such a switch automatically shifts the light source into the passive operational mode and simultaneously actuates the voltage supply to the heating element, while at weak illumination, automatically shifts the light source into the active operational mode thereof the heating element being thereby disconnected